I. Field of the Invention
This invention relates to stripper rubbers used to seal against a drill string deployable through a drilling head and, in particular, to a reinforced stripper rubber for preventing extrusion of the stripper under pressure and reducing the tensile and compressive stresses on the stripper rubber.
II. Description of the Prior Art
Drilling heads are widely used in drilling operations to prevent the drilling fluids from flowing out onto the rig floor. The typical drilling head incorporates a diverter passageway for diverting the drilling fluids, a rotatable drive assembly with bearings to rotate with or drive the drill string, and a stripper rubber positioned within the drilling head. The stripper rubber may be secured to the rotating drive assembly in order to rotate with the drill string and seal against the string to prevent drilling fluids from flowing up through the drilling head. The stripper rubber must be resilient enough to allow various tools and couplings to pass through the drilling head while maintaining the seal. More recent drilling heads have incorporated two axially separated stripper rubbers to maintain the seal thereby preventing fluid flow through the drilling head.
Prior known drilling heads have incorporated stripper rubbers of varying sizes and shapes in order to maintain the desired sealing engagement under the fluid pressures found in drilling operations. The single greatest disadvantage of prior known stripper rubbers is their tendency to extrude and invert under extreme pressures. In the typical assembly, the stripper rubber is mechanically restrained only by the insert at the top of the drilling head. As a tool joint of the drill pipe is lowered through the drilling head, the stripper rubber is pulled in tension and elongates severely until the tool joint has passed through the rubber. Such stretching and elongation results in high stress loads on the stripper rubber. When fluid pressure is applied to the stripper rubber, the rubber is loaded in compression. Since the lower portion of the stripper is unsupported, the rubber can extrude and invert. This problem is exacerbated as a drill string joint is being drawn upwardly through the stripper rubber.
Attempts to eliminate inversion of the stripper rubber have included increasing the strength of the materials and the thickness of the stripper rubber. However, such changes affect the resiliency of the stripper rubber and therefore the ability to maintain the proper seal against the varying thicknesses of the drill string.
Different stripper rubbers have been developed for different applications according to the environment and workload placed upon the rubber. "Shorty" stripper rubbers have been developed for use in minimal clearance drilling heads. High Pressure and Heavy Duty stripper rubbers are used in specialized applications. Thus, a single stripper rubber capable of use in the various drilling heads is needed.